The problem of electrostatic discharge (ESD) is well known. From merely receiving a mild shock after crossing a room and touching a metal object, to sending a shock into electronic equipment, nearly everyone has experienced an ESD problem at some time.
While static electricity is extremely complex, several overall theories are generally accepted with regard to the action of ESD. Static electricity charges on a person or object are generally like charges (that is, the charges are all positive or all negative). As such, as static electricity charges build up on a person or object, these charges tend to migrate as far apart from each other as possible as determined by the geometry of the person of object. Thus, for example, it is common for static electricity charges to migrate to a person's fingertips. For this reason, when that person reaches out to touch an electrically conductive object, a spark will jump the gap between that person's fingertips and the object based upon the potential difference between the fingertips and the object. This discharge is very rapid and can be quite violent. If the electrically conductive object is sensitive electronic equipment, that equipment may be damaged either from the magnitude of the discharge and/or from the speed of the discharge. At the least, the charge could cause the equipment to execute an error. A sufficient number of such discharges may eventually damage the equipment.
Accordingly, the art contains many inventions intended to protect equipment or a person from the effects of this sudden, and sometimes violent, discharge associated with ESD.
For instance, in the logging industry where chains are lowered by helicopter to loggers waiting on the ground to fasten fresh-cut timber to them so it could be airlifted to the sawmill or nearby waterway, track access point or the like, the loggers are often reluctant to grab the chain because of a painful shock that may occur as a result of a buildup of static electricity which will be discharged to ground through their bodies. This particular problem has been solved by incorporating a resistance in the line from the charge-carrying object, such as the helicopter, to the person on the ground. The high resistance causes the current to be low enough that the discharge will not be painful.
However, it may be cumbersome or inconvenient to include a resistance in the line itself. This solution may be even more cumbersome if the person is an office worker who moves around a great deal. Accordingly, this solution to the ESD problem has serious shortcomings.
Accordingly, there is a need for a system that protects a person against the effects of ESD but can do so in a manner that does not interfere with any task the person may be performing and further will not be cumbersome or burdensome for the person to use.
While many devices currently on the market work well, there are several problem areas not addressed thereby. This results in drawbacks and disadvantages for such devices when a person or equipment are situated in certain environments or subject to certain conditions.
First, no matter how effective a touch pad is it will be totally ineffective if the person does not use it. That is, if the person carrying a large ESD charge forgets to touch the touch pad and proceeds to touch a computer, the ESD will discharge through the computer and the touch pad will have been useless. Thus, a shortcoming of such touch pads is that they require the person to remember to use it.
Furthermore, no matter how effective the ESD protection device is, the current level and/or the change in current level may be so high that either the person or the equipment can be damaged.
Still further, while placing a touch pad on a computer may protect the computer it does not protect the user from the effects of an electrostatic discharge.
As mentioned above, the majority of applications for the prevention of ESD are in the manufacturing or medical fields and are largely concerned with protecting the ‘manufacturing’ process or sensitive components for ESD damage. Examples include moving mediums such as the manufacture of rolls of paper, the assembly of delicate electronic chips and circuitry and surgeon-patient contact during an operation.
An analysis of each of the above will help illustrate the shortcomings of the prior art. In the manufacture or printing of paper, long rolls of paper may move at high speed. Often the path may involve rubber or other rollers and guides. As the paper rubs across such items a static electricity charge may be generated. Since the paper path is well controlled, it is an easy process to place grounded conductive brushes or flat metal springs in contact with the moving paper since the paper stays in a fixed path. Such electrodes are connected directly to the grounded frame of the associated machinery or to another path eventually leading to earth ground or other equalizing means.
Another common application of ESD control is in the production or repair of fragile electronics such as computer circuit boards. Even a slight electrostatic discharge through a sensitive device may destroy it. Therefore, significant effort and cost is devoted to eliminating the possibilities of electrostatic potentials in the vicinity of the sensitive electronics. Typically, a single ground point is provided that all associated elements are connected to so that no electrostatic potential can exist between them that might flow through the sensitive electronics. For example, a conductive floor mat is provided that is connected to the ground point, or a work surface mat that is conductive (or dissipative) is also provided that is wired to the same ground point, the work table frame and any test equipment is connected to the same point, finally the assembly person is also connected to the same point, typically by a wrist strap tether. The tether generally consists of a wrist pad and grounding wire that is eventually connected back to the single ground point. For operator safety, the ground wire typically contains a 1 Meg resistor to limit current flow to safe levels should the operator come in contact with 120 volts AC. This tethering restraint is inconvenient and not considered suitable for a typical office worker or call center operator. The single ground point is eventually connected to true earth ground or other equalizing point by another conductor.
Applications are similar in the medical field, employing similar tethers and/or foot/shoe connectors also considered impractical for the typical office worker environment.
Today, a new set of ESD problems is emerging in the typical work place or home office environment. Today, a typical worker may exist in a virtually electrically isolated environment—a plastic computer case, plastic keyboard, plastic control knobs on a molded plastic control panel, plastic office chair with man-made fabric and plastic wheels, non-conductive flooring or carpeting and even a headset with foam or molded plastic earpieces and plastic microphone tube.
As the operator moves in his/her chair, there are many opportunities for a very large electrostatic charge to build up on his/her body. Friction between dissimilar materials is the classical means for generation of electrostatic voltages. There are many such situations that exist continually in the operator environment today—the operator's clothing sliding against the chair back or arm rests, the operator's shoes sliding on the carpet, the plastic chair wheels sliding against the carpet are a few examples. The effects can be cumulative over a long period of time, and can become quite high.
Eventually a discharge or equalization to (true earth) ground must take place. The higher the value of the electrostatic voltage charge, the greater the distance the charge may ‘jump’ to discharge, and the more ‘catastrophic’ the event to the operator. This is an example of corona discharge. For example, there are many documented cases of operators in call centers experiencing a very loud pop or explosion in their ear, ear pain, and even bleeding in the ear as the discharge path appears to take place through the operator's headset. Other documented cases include severe neck pain, nausea, numbness, elevated blood pressure and rapid heart beat.
There are many possibilities as to why these effects are worse than the typical nuisance static electricity charges walking around the house. For instance, the discharge path may be more surprising or appear worse to the user if it involves the user's ear. Recently, this has been attributed to electrostatic discharge of the operator with the grounding mechanism being the metallic portion of the ear piece coupled to its metallic conductors and eventually to earth ground through its associated electronics. This may be a direct low impedance ground or it might be a higher impedance which is still sufficiently low with respect to that needed to successfully equalize the static charge. Still in other cases, as explained below, the associated electronics may potentially make the discharge injury to the person more severe and disturbing by causing a high current pulse to take place as the discharge event. This effect may be further compounded by allowing the operator to be exposed to other voltage or leakage paths developed via the ear over time.
In some cases, the associated electronics may experience physical damage or processing disturbances due to the operator electrostatic discharge. For example, the headphone circuit might involve a transformer with a 600 to 1000 volt breakdown rating between its windings (connected to the headset diaphragm) and conductive metal core. However, the electrostatic voltage on the operator may exceed 15,000 volts—far more than the design tolerance of the transformer. Should the transformer be exposed to such excessive high voltage, a ‘breakdown’ or ‘shorting’ may occur. Thus, the operator electrostatic voltage might cause a ‘short circuit’ insulation breakdown or lower resistance to develop between the headset winding (secondary) and primary winding which may be at a constant high voltage level with respect to ground or the transformer core which may be connected to earth ground, thus completing the discharge path.
The transformer breakdown may cause a permanent equipment failure. Other equipment damage or errors can also occur due to the electrostatic discharge event. The electrostatic discharge event may cause an electromagnetic or radio frequency pulse to be generated. This pulse may radiate or migrate into nearby circuitry causing errors in processing or noise in audio or video circuits. Although a transformer discharge event has been described above, other similar discharge paths can be envisioned, with similar catastrophic results. Since many electronic devices are connected together by some path, including a ground path, it is easy to understand from the foregoing that a pulse or an ESD event in one area can have consequences in other areas as well unless the event is controlled and contained.
With continued miniaturization of electronics, the problems may become more severe as circuit component voltage tolerances become less and enclosure insulation distances become less.
Accordingly, there is a need for an ESD protection system that protects a person and electronics from the effects of ESD, even if that person is in an environment that is intended to nominally insulate that person from ground.
As the cost of doing business increases, many businesses are reluctant to purchase new equipment. Thus, it is most advantageous if existing equipment can be easily modified or retrofit to achieve new and improved results. This is the situation with protecting people from the effects of ESD. Thus, there is a need for a system for protecting people against the effects of ESD that can easily be retrofit onto existing equipment.
There are also other needs for protection from ESD. As new wiring is being installed by being pulled through raceways and conduits, an electrostatic charge may be generated along the cable. This may cause arc-over damaging the cable itself, damage to equipment when connected to it, shock to the person installing the cable and latent damage. Fire and explosions in gas stations during fuel purchase are also a similar ESD problem.
An overall system for protecting a person or equipment from the effects of ESD is disclosed in patent application Ser. No. 09/934,047 which is now U.S. Pat. No. 6,873,516 and in application Ser. No. 10/112,952, which is now U.S. Pat. No. 6,987,659. The disclosures of these two documents are incorporated herein by reference thereto.
Important aspects of any circuit element, no matter what its application, are ease of use and the ease with which the element can be designed or modified to accommodate a particular situation. That is, it is important that a circuit element be easily modified to account for factors that may be influenced by the particular situation.
Accordingly, while the present invention is being disclosed in conjunction with ESD, those skilled in the art will understand, based on the teaching of this disclosure, that the invention has applications that are much broader than the ESD field. For example, the invention can be used anywhere an element having high resistance can be used or is required without departing from or exceeding the scope of the present disclosure.
For purposes of this disclosure, it is noted that the term “dissipative” will refer to resistance values in excess of 107 ohms and can be as high as 1012 ohms.